Which is it Lemmy?

Comments

• NeatNit@discuss.tchncs.de ⁨2⁩ ⁨months⁩ ago

  The answer is definitely B (see other comments), but I think the more interesting question is, how does it affect the portals themselves and the surfaces they’re attached to?

  When the portals are attached to a solid unmovable wall (as in the games), we can pretty much ignore the forces acting upon the wall as they will just be automatically cancelled out by the normal force. But the same does not apply for portals attached to detached panels. Let’s say that instead of a piston pushing down, the portal is attached a free-falling panel with the portal facing down.

  We are inclined to assume that as the portal falls onto the cube, it feels no force, as if it fell through air or through a vacuum. But is that really the case? Maybe instead, the falling portal experiences resistance from the cube? Or even, actually, an accelerating force?

  Let’s assume conservation of momentum at each end of the portal. That is, in the general case: if the portal (which we assume has no mass) is attached to an object with mass M and velocity V, and “swallows” a cube or other object with mass m and velocity v, and at the end of this interaction the portal-surface object with mass M has velocity V’, then we expect:

  M × V’ = (M × V) + (m × v)

  Mass is not conserved because the cube was transferred to the other portal.

  Solve for V’:

  V’ = V + (m/M) v

  This is a strange result. The cube’s momentum is imparted into the portal, but the result would be different depending on our reference frame. From a reference frame tracking the cube pre-interaction, v = 0 (by definition) and so V’ = V, the portal-surface object has no change in velocity. But in any other reference frame, it does.

  Physics can’t be affected by reference frame (thanks, relativity) so this analysis must be wrong.

  … I’ll keep thinking on this.

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  • NeatNit@discuss.tchncs.de ⁨2⁩ ⁨months⁩ ago

    Okay, after thinking about this a while, I’ve decided that conservation of momentum just doesn’t work, unless the portal actually absorbs the mass of every object passing through it, which would probably cause its own set of contradictions.

    Instead, I think forces that act through the portal are applied to the portal. What do I mean by this? Well, take this classic case of a cube floating halfway through two floor portals:

          ____            ____
         |    |          |    |
    ___++++++++++______----------___
    

    (sorry, can’t do better than ASCII art right now)

    The system has reached equilibrium, and nothing is moving. Both halves of the cube are being pulled down by gravity, but cancel each other out by applying a normal force on each other right at the portal interface. What I’m claiming is that this normal force is applied to the portals, which ultimately transfer it to the floor. If this entire floor was a scale, it would measure the calibrated weight of the Weighted Storage Cube.

    But this is the static analysis. What happens when objects are in motion? To be continued…

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    • NeatNit@discuss.tchncs.de ⁨2⁩ ⁨months⁩ ago

      So we might think: a cube falling through a floor portal and flying out another floor portal right next to it, kinda looks like how a bouncy ball bounces off the floor, and should apply forces on the floor similar to that - an elastic collision. Unfortunately, this brings us right back to conservation of momentum, which didn’t work out. But maybe I just did it wrong? Maybe, if done more carefully, conservation of momentum can work without being dependent on the reference frame?

      I genuinely don’t know yet. Time to bring out the calculus.

      We have:

      M = the mass attached to the portal (e.g. the floor)

      V = the velocity of M

      V’ = dV/dt = the acceleration of M (assuming no other forces)

      F = MV’ = the force applied on the portal

      m’ = dm/dt = the rate of mass entering the portal

      v = the velocity of the mass entering the portal

      If we assume conservation of momentum (our old nemesis), the force applied on the portal should be the same as the momentum taken from the mass entering the portal:

      F = m’v

      But now we’re back to the case from before, where the choice of reference frame changes the outcome! We’re not having that. So I hereby decree:

      The force applied upon a portal by a mass entering it must be calculated by conservation of momentum in the intertial reference frame of the portal itself at the instant under examination.

      This gives us, effectively: F = m’(v-V)

      Now THAT’S something we can work with!

      Note that this formula also works for the exit portal. In that case, m’ is negative.

      A curious result follows.

      For the entry portal, m’ is positive, and (v-V) is a vector pointing into the portal. (This must be the case, otherwise the mass would be exiting the portal). Therefore, the force would be pushing the portal into the wall.

      For the exit portal, m’ is negative, and (v-V) points out of the portal, therefore the portal again would be pushed into the wall.

      The result: whenever anything passes through a portal, both ends always get pushed in the same direction.

      So, back to my thought experiment.

      If a portal attached to a free-falling panel falls onto a cube, the cube’s entry into it would brake its fall to some extent. However, this wouldn’t be the only force in play.

      As I said in my previous comment, forces applied between the two halves of an object going through a portal, get applied to the portal itself. In practice, this mostly means tension and compression forces. In the static analysis from before, it was compression.

      As the panel is falling onto the cube, a part of the cube is already out the other side and moving away the portal. If the panel has slowed, this would apply a pulling force (tension) on the part of the cube that still hasn’t made it in. This tension force actually compels the portal, and the panel it’s attached to, to speed up towards the ground.

      So, in total, would the panel be slowed, accelerated, or unaffected? Maybe the forces cancel out and it just continues on its normal freefall? I don’t know yet. Would need to do more math.

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• empireOfLove2@lemmy.dbzer0.com ⁨2⁩ ⁨months⁩ ago

  A. Object momentum relative to the world is retained thru portals regardless of the speed of the portal surface or the portal itself. The portal is only transmitting matter from A to B, it does not impart energy of any kind.

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  • FishFace@piefed.social ⁨2⁩ ⁨months⁩ ago

    If the portals don’t impart energy, you wouldn’t gain speed when you put one directly above another and drop through forever. The higher portal increases your potential energy.

    What if the entire room were rigged to a piston, the orange portal were held still and the room forced upwards at speed. Would you see scenario B then?

    If so, how do the portals know which to choose? Remember that neither portal is actually stationary: they’re both orbiting the sun at 30km/s, etc.

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    • empireOfLove2@lemmy.dbzer0.com ⁨2⁩ ⁨months⁩ ago

      Its not the portal doing that though. It’s gravity. Your own momentum moves you through the portal while gravity is acting upon you; the portal is nothing but a doorway.

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• Marty_TF@lemmy.zip ⁨2⁩ ⁨months⁩ ago

  a. while the portal travels down it collects air, increasing air pressure on the other side, meaning once it reaches the cube, the increased air pressure has mad the air solid so the cube cant fly through it.

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  • NeatNit@discuss.tchncs.de ⁨2⁩ ⁨months⁩ ago

    Now there’s a high quality ragebait answer

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• ButteredBread@sh.itjust.works ⁨2⁩ ⁨months⁩ ago

  Ignoring the game it would in theory be B because it would need to leave the portar at the relative speed it entered it.

  For A to happen it would need to leave slowly which is impossible because it entered quickly and keeps momentum.

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  • YoFrodo@lemmy.world ⁨2⁩ ⁨months⁩ ago

    [deleted]

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    • ButteredBread@sh.itjust.works ⁨2⁩ ⁨months⁩ ago

      speed is relative.

      Ignoring that fact tho. it would still exit at the speed the pressure moved. if you say it was static it should not exit at all sonce speed was 0.

      Another way to see it is also how every chunk that enters the portal needs to be on the other side instantly. So it would need to exit at the speed the portal moves because if it didn’t not it wpuld not get to the other side instantly and then again, it should keep momentum.

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    • ThatWeirdGuy1001@lemmy.world ⁨2⁩ ⁨months⁩ ago

      If the universe moved around you would other people not perceive you moving? Would that not be indistinguishable to movement?

      Image

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  • luthis@lemmy.nz ⁨2⁩ ⁨months⁩ ago

    This is excellent rage bait

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    • ButteredBread@sh.itjust.works ⁨2⁩ ⁨months⁩ ago

      wut

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• Fedegenerate@fedinsfw.app ⁨2⁩ ⁨months⁩ ago

  “Speedy thing goes in, speedy thing comes out” as the cube isn’t a speedy thing (no speed lines) going in, it won’t be a speedy thing coming out.

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• silver_wings_of_morning@feddit.dk ⁨2⁩ ⁨months⁩ ago

  The real answer is the enemies we made along the way

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• muusemuuse@sh.itjust.works ⁨2⁩ ⁨months⁩ ago

  A

  In this example, the object wasn’t moving. It has no momentum to continue moving. Instead, the world around it moved. So A

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  • Honytawk@discuss.tchncs.de ⁨2⁩ ⁨months⁩ ago

    Movement is relative.

    You need a second reference point for movement to exist.

    To the (for us) moving portal, the cube was moving. And it would keep that momentum.

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    • CaptPretentious@lemmy.world ⁨2⁩ ⁨months⁩ ago

      Get a hula hoop attach a camera to it. Drop hula hoop over something with it being in the middle. Did thing in hula hoop take off? No. Thing had no momentum. It’s velocity versus relative velocity.

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    • iocase@lemmy.zip ⁨2⁩ ⁨months⁩ ago

      Yeah if this existed IRL B is the correct answer. If you tried this in portal the video game A is the answer.

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  • TheColonel@reddthat.com ⁨2⁩ ⁨months⁩ ago

    Correct.

    It’s like dropping a hula hoop over it in this scenario. There is nothing to generate its motion.

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    • iocase@lemmy.zip ⁨2⁩ ⁨months⁩ ago

      Using portals rules yeah A is the correct answer.

      B would be correct if this existed in real life and had to be compatible with general relativity. If we took this exact same setup and moved it into space, how could you tell if the cube is moving towards the portal or the portal is moving towards the cube? You also can’t use the blue portal as a reference point since it could also be moving indipendantly from the orange portal.

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• NeatNit@discuss.tchncs.de ⁨2⁩ ⁨months⁩ ago

  The “ragebait answers only” in the OP is the only thing stopping me from actually raging at everyone saying A. I am giving them all the benefit of the doubt, they’re deliberately saying the wrong answer.

  Everyone saying B missed the assignment and answered accurately.

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  • ThatWeirdGuy1001@lemmy.world ⁨2⁩ ⁨months⁩ ago

    Where do the facts begin and the rage ragebait ends? Who knows, that’s the fun.

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• Fedizen@lemmy.world ⁨2⁩ ⁨months⁩ ago

  The piston moves throught the portal and the portal hovers in the air.

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• ephrin@sh.itjust.works ⁨2⁩ ⁨months⁩ ago

  A

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• iocase@lemmy.zip ⁨2⁩ ⁨months⁩ ago

  This is Einstein’s elevator thought experiment but with portals.

  B

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  • Azrael@reddthat.com ⁨2⁩ ⁨months⁩ ago

    Einstein’s elevator explains why acceleration can mimic gravity. It doesn’t, by itself, explain why a stationary cube would emerge with a large sideways velocity. That conclusion depends on additional assumptions about how moving portals transfer momentum.

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    • iocase@lemmy.zip ⁨2⁩ ⁨months⁩ ago

      Yeah it does explain it if you use your brain for like half a second. Is the orange portal moving down to the cube or is the cube moving up towards the orange portal? We have the reference frame of the cube so it looks like the portal is approaching the cube.

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• ThatWeirdGuy1001@lemmy.world ⁨2⁩ ⁨months⁩ ago

  Ragebait aside the answer is B.

  If you had a table with a cylinder, with a ping pong ball in it, and a circular weight dropped around the top of the cylinder that hits the table, the ping pong ball will bounce due to the forces of the weight hitting the table. Enough energy would be transferred through the impact, that would cause the ball to seemingly jump up in the cylinder.

  Physics are weird.

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  • mrgoosmoos@lemmy.ca ⁨2⁩ ⁨months⁩ ago

    nice ragebait answer, but I’m not falling for it

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• arotrios@lemmy.world ⁨2⁩ ⁨months⁩ ago

  A if it’s a cube. B if it’s beans.

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• PotatoPie@lemmy.zip ⁨2⁩ ⁨months⁩ ago

  A 🔥

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• zephiriz@lemmy.ml ⁨2⁩ ⁨months⁩ ago

  Was going to post the correct Answers (A) but then looked at the post and though na. But if you want to know more about portals and how they would work in real life check this out. Great guy.

  youtu.be/1zgBwQVr-wI

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  • flying_sheep@lemmy.ml ⁨2⁩ ⁨months⁩ ago

    and his answer: optozorax.github.io/portal/?scene=speed_model

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• BlackLaZoR@lemmy.world ⁨2⁩ ⁨months⁩ ago

  It’s a superposition of both states until you make a measurement

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  • ThatWeirdGuy1001@lemmy.world ⁨2⁩ ⁨months⁩ ago

    Image

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• FreddiesLantern@leminal.space ⁨2⁩ ⁨months⁩ ago

  B

  The cube gets sucked into the portal with the same force the platform in A is getting sucked upwards.

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  • Azrael@reddthat.com ⁨2⁩ ⁨months⁩ ago

    It would be A because it’s the orange portal that’s moving, not the cube.

    The only way it would be B is if the portals suck since the cube doesn’t have any momentum to begin with. The game clearly shows that this is not how portals work.

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    • EvilHankVenture@lemmy.world ⁨2⁩ ⁨months⁩ ago

      Think about how you come out of the second portal, your head comes out first and the rest of your body follows. Your head has to have a velocity coming out of the portal or the rest of your body will smash into you. So either you come out flying or you’re smashed to a pulp.

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• markz@suppo.fi ⁨2⁩ ⁨months⁩ ago

  A doesn’t work.

  So you’re going really fast through the portal until the point you’ve fully passed it, at which you somehow decelerate or stop?

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• HailHydra@infosec.pub ⁨2⁩ ⁨months⁩ ago

  A youtu.be/DydIhwLrbMk?t=1561

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• Crozekiel@lemmy.zip ⁨2⁩ ⁨months⁩ ago

  You can’t put a portal on a moving platform, it has to stop moving first, so neither can happen… Unless the moving surface is a celestial body at least the size of The Moon, I guess?

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  • Mediocre_Bard_Redeux@lemmy.world ⁨2⁩ ⁨months⁩ ago

    What!? What if the platform is a train on Earth?

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• eah@programming.dev ⁨2⁩ ⁨months⁩ ago

  I reject your reality and substitute my own.

  The correct answer is C: the cube plops out and the object to which the blue portal is affixed flies off in the southwest direction.

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• Duamerthrax@lemmy.world ⁨2⁩ ⁨months⁩ ago

  B.

  As the cube enters the portal, it will take on energy from the piston, slowing the piston down in the process, and that energy will propel it out. If it enters the portal fast, it has to exit the portal fast.

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  • BitUnWise@programming.dev ⁨2⁩ ⁨months⁩ ago

    It pushes off of the bottom platform, the cube accelerates by pushing off of itself (if it didn’t it would flatten into a pancake). And because there’s a downward force on the bottom of the cube it pushes down on the platform

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    • Duamerthrax@lemmy.world ⁨2⁩ ⁨months⁩ ago

      I’m not sure what you’re saying. You mean the piston and portal would compress the platform? Even if that’s the case, that still means force is being transferred through the portal.

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  • Bytemeister@lemmy.world ⁨2⁩ ⁨months⁩ ago

    As the cube enters the portal, it will take on energy from the piston

    Why would it take the energy of the piston? It never interacts with the piston at all.

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    • Duamerthrax@lemmy.world ⁨2⁩ ⁨months⁩ ago

      Regardless of the how the portal works, if the cube enters the portal at a certain rate, it has to exit the portal at the same rate and everything else has to follow.

      The cube doesn’t interact with the piston, but it does interact with the portal and the portal is fixed to the piston. The same way that when I hit a nail with a hammer, I feel the resistance of the nail though the hammer handle. I don’t touch the nail, but the force is transferred through the hammer and in the hypothetical, the force would be transferred to the portal.

      The hypothetical does not bring up the possibility that the portal has infinite sharpness at it’s edges and would cut though the piston head or the possibility that relativity is somehow involved and would allow the cube to exit the portal at a different rate that it enters. Wormholes moving at different rates is a theoretical form of time travel, but not at these speeds.

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• ramble81@lemmy.zip ⁨2⁩ ⁨months⁩ ago

  Now what happens if you put it on a treadmill?

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• Th4tGuyII@fedia.io ⁨2⁩ ⁨months⁩ ago

  C. The moving portal platform breaks the black platform and the whole thing goes through the portal.

  ---

  Jokes aside, B.

  Devoid of any other external forces, in the cube's reference frame there's functionally no difference between it moving towards the portal or the portal moving towards it at a constant acceleration.

  Ergo the cube should be pushed out the other side at the same momentum that the system had prior to the collision, and not be simply plopped out as in answer A.

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• moakley@lemmy.world ⁨2⁩ ⁨months⁩ ago

  The top of that masher is clearly protecting the cube from the rain, so I’m not sure how the rain is expected to shoot the cube out as in option B.

  A.

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• whereitsat@lemmy.zip ⁨2⁩ ⁨months⁩ ago

  you know only engineer/computer science nerds post on lemmy because nobody has called out that the box looks like the box from hellraiser.

  no matter what you do to it, you’ll probably end up in hell.

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• prime_number_314159@lemmy.world ⁨2⁩ ⁨months⁩ ago

  It depends. It looks like the orange portal is being moved by a hydraulic cylinder (aka laminar flow), which means the relative positions and velocities of things moving through the orange portal are conserved, and A is correct.

  If instead that’s (yuck) an air fired cylinder… Then it’s going to be B.

  If the movement is handled by a threaded rod, then the box will emerge from the blue portal spinning, and (perhaps contrary to expectations) if the orange portal is moving because of gravity, then the bouyant force of the box progressing through the portal will stop it from falling further, and the box will end up just less than halfway (because of the angle) through the blue portal.

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  • iocase@lemmy.zip ⁨2⁩ ⁨months⁩ ago

    And as a matter of fact the fastest possible velocity you can get is with a hydraulic cylinder because of the immense amount of force it applies using the orange portal.

    If you used magnetic levitation to move the orange portal the box would never make it through, because it’s made of metal and will be attracted to the levitation equipment.

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• Gigdragon@lemmy.world ⁨2⁩ ⁨months⁩ ago

  This is clearly an unbirth scenario and the A and B things are unrelated. Either way, your baby is companion cube and GladOs is the father.

  Thanks for coming to my Ted X talk.

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• subignition@fedia.io ⁨2⁩ ⁨months⁩ ago

  The entire pedestal the cube is on comes through the blue portal and the cube slides to the floor

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• offspec@lemmy.world ⁨2⁩ ⁨months⁩ ago

  There’s a guy on YouTube with excellent portal simulations that’s been working on the theory of portal physics for a while named optozorax

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